None of that matters because your data needs to be modeled around your application usage patterns. If you maintain inter-collection relationships via Joins in Mongo just as if you were in Postgres, you're not doing it right.
Your schema needs to be modelled around your domain, because that will dictate what your application usage patterns actually are. That will determine wether using a document database is feasible or not. If not, then there isn’t any design cleverness that can save you.